The past decade has seen a tremendous growth in the amount of data traffic carried over wide area networks. This trend is expected to continue with the growing popularity of the Internet and the emergence of new applications such as Voice-over-Internet Protocol (VoIP).
In addition to the rising traffic volumes, there has also been an evolution in the underlying networks. From the optical transport layer and up, network topologies have become more mesh-like, allowing multiple diverse paths between source and destination nodes. This diversity is essential in providing resiliency for critical demands via backup paths. A high degree of connectedness in a network also allows sharing of traffic load across various links and demands, and hence better network utilization. This is important because long-haul bandwidth continues to be expensive due to the high costs of Wavelength Division Multiplexing (WDM) transport systems and high speed router ports.
Load-sharing can be achieved in two complementary ways. One way is through congestion-aware routing algorithms to route the demands such as, for example, Open Shortest Path First (OSPF) techniques. Another way is by routing the packets of the same demand over multiple paths along the way. The latter, called multi-path routing, provides fast resiliency as well as a finer degree of load sharing in the network. In fact, OSPF and other approaches have multi-path extensions, such as equal-cost multipath (ECMP) and optimized multipath (OMP), where routers distribute the incoming load on an interface over all available shortest paths. It is to be understood that “shortest paths” generally refers to the cheapest paths under the cost metric chosen by the OSPF algorithm.
In multi-path routing, packets can be distributed using either a round-robin mechanism or a hash function on the flow identifiers. The hash-based approach routes all packets of a flow over the same path and may lead to load imbalances due to variations in flow rates. While the round-robin scheme will lead to better load sharing, since the packets of the same flow may be sent over different links, they can arrive out-of-order at the destination. If not resequenced, out-of-order arrival of packets leads to increased dropping of packets by the higher layer protocols (e.g., Transmission Control Protocol), as well as jitter in delay. Resequencing at high traffic rates, on the other hand, requires expensive processors and large memories. As a result, the round-robin mechanism has been mostly unused in practice.
Admission control and capacity planning in a network require an accurate knowledge of the bandwidth needed on each link to carry the given traffic load. However, it is difficult to exactly compute the bandwidth needs of variable bit-rate (VBR) traffic, such as most of the data traffic. This is typically handled in practice via the concept of “effective bandwidth,” which is an estimate of the bandwidth needed to satisfy a quality-of-service (QoS) requirement such as, for example, a drop rate, a maximum queuing delay, etc. Effective bandwidth depends on the traffic characteristics, i.e., the average rate and the variability, as well as the strictness of the QoS requirement.